M.M.M. Meier^a, B. Schmitz^b, A. Lindskog^a, C. Maden^c, R. Wieler^c

^aLund University, Department of Geology, Sölvegatan 12, SE-22362 Lund, Sweden
^bLund University, Department of Physics, SE-22100 Lund, Sweden
^cETH Zurich, Department of Earth Sciences, CH-8092 Zurich, Switzerland

We measured the He and Ne concentrations of 50 individual extraterrestrial chromite grains recovered from mid-Ordovician (lower Darriwilian) sediments from the Lynna River section near St. Petersburg, Russia. High concentrations of solar wind-like He and Ne found in most grains indicate that they were delivered to Earth as micrometeoritic dust, while their abundance, stratigraphic position and major element composition indicate an origin related to the L chondrite parent body (LCPB) break-up event, 470 Ma ago. Compared to sediment-dispersed extraterrestrial chromite (SEC) grains extracted from coeval sediments at other localities, the grains from Lynna River are both highly concentrated and well preserved. As in previous work, in most grains from Lynna River, high concentrations of solar wind-derived He and Ne impede a clear quantification of cosmic-ray produced He and Ne. However, we have found several SEC grains poor in solar wind Ne, showing a resolvable contribution of cosmogenic ²¹Ne. This makes it possible, for the first time, to determine robust cosmic-ray exposure (CRE) ages in these fossil micrometeorites, on the order of a few hundred-thousand years. These ages are similar to the CRE ages measured in chromite grains from cm-sized fossil meteorites recovered from coeval sediments in Sweden. As the CRE ages are shorter than the orbital decay time of grains of this size by Poynting-Robertson drag, this suggests that the grains were delivered to Earth through direct injection into an orbital resonance. We demonstrate how CRE ages of fossil micrometeorites can be used, in principle, to determine sedimentation rates, and to correlate the sediments at Lynna River with the fossil meteorite-bearing sediment layers in Sweden. In some grains with high concentrations of solar wind Ne, we nevertheless find a well-resolved cosmogenic ²¹Ne signal. These grains must have been exposed for up to several 10 Ma in the regolith layer of the pre-break-up L chondrite parent body. This confirms an earlier suggestion that such regolith grains should be abundant in sediments deposited shortly after the break-up of the LCPB asteroid.

Reference
Meier MMM, Schmitz B, Lindskog A, Maden C and Wieler R (in press) Cosmic-ray exposure ages of fossil micrometeorites from mid-Ordovician sediments at Lynna River, Russia. Geochimica et Cosmochimica Acta
[doi:10.1016/j.gca.2013.10.026]
Copyright Elsevier

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E. Gnos^1,*, B. A. Hofmann², M. A. Halawani³, Y. Tarabulsi³, M. Hakeem³, M. Al Shanti³, N. D. Greber^2,4, S. Holm⁵, C. Alwmark⁵, R. C. Greenwood⁶, K. Ramseyer⁴

¹Natural History Museum of Geneva, Geneva 6, Switzerland
²Natural History Museum Bern, Bern, Switzerland
³Saudi Geological Survey, Jeddah, Saudi Arabia
⁴Institute of Geological Sciences, University of Bern, Bern, Switzerland
⁵Department of Geology, Lund University, Lund, Sweden
⁶Planetary and Space Sciences, The Open University, Milton Keynes, UK

The very young Wabar craters formed by impact of an iron meteorite and are known to the scientific community since 1933. We describe field observations made during a visit to the Wabar impact site, provide analytical data on the material collected, and combine these data with poorly known information discovered during the recovery of the largest meteorites. During our visit in March 2008, only two craters (Philby-B and 11 m) were visible; Philby-A was completely covered by sand. Mapping of the ejecta field showed that the outcrops are strongly changing over time. Combining information from different visitors with our own and satellite images, we estimate that the large seif dunes over the impact site migrate by approximately 1.0–2.0 m yr⁻¹ southward. Shock lithification took place even at the smallest, 11 m crater, but planar fractures (PFs) and undecorated planar deformation features (PDFs), as well as coesite and stishovite, have only been found in shock-lithified material from the two larger craters. Shock-lithified dune sand material shows perfectly preserved sedimentary structures including cross-bedding and animal burrows as well as postimpact structures such as open fractures perpendicular to the bedding, slickensides, and radiating striation resembling shatter cones. The composition of all impact melt glasses can be explained as mixtures of aeolian sand and iron meteorite. We observed a partial decoupling of Fe and Ni in the black impact glass, probably due to partitioning of Ni into unoxidized metal droplets. The absence of a Ca-enriched component demonstrates that the craters did not penetrate the bedrock below the sand sheet, which has an estimated thickness of 20–30 m.

Reference
Gnos E, Hofmann BA, Halawani MA, Tarabulsi Y, Hakeem M, Al Shanti M, Greber ND, Holm S, Alwmark C, Greenwood RC and Ramseyer K (in press) The Wabar impact craters, Saudi Arabia, revisited. Meteoritics & Planetary Science
[doi:10.1111/maps.12218]
Published by arrangement with John Wiley & Sons

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Indrani Banerjee and Banibrata Mukhopadhyay

Department of Physics, Indian Institute of Science, Bangalore 560 012, India

We investigate nucleosynthesis inside the outflows from gamma-ray burst (GRB) accretion disks formed by the Type II collapsars. In these collapsars, massive stars undergo core collapse to form a proto-neutron star initially, and a mild supernova (SN) explosion is driven. The SN ejecta lack momentum, and subsequently this newly formed neutron star gets transformed to a stellar mass black hole via massive fallback. The hydrodynamics and the nucleosynthesis in these accretion disks have been studied extensively in the past. Several heavy elements are synthesized in the disk, and much of these heavy elements are ejected from the disk via winds and outflows. We study nucleosynthesis in the outflows launched from these disks by using an adiabatic, spherically expanding outflow model, to understand which of these elements thus synthesized in the disk survive in the outflow. While studying this, we find that many new elements like isotopes of titanium, copper, zinc, etc., are present in the outflows. ⁵⁶Ni is abundantly synthesized in most of the cases in the outflow, which implies that the outflows from these disks in a majority of cases will lead to an observable SN explosion. It is mainly present when outflow is considered from the He-rich, ⁵⁶Ni/⁵⁴Fe-rich zones of the disks. However, outflow from the Si-rich zone of the disk remains rich in silicon. Although emission lines of many of these heavy elements have been observed in the X-ray afterglows of several GRBs by Chandra, BeppoSAX, XMM-Newton, etc., Swift seems to have not yet detected these lines.

Reference
Banerjee I and Mukhopadhyay B (2013) Nucleosynthesis in the Outflows Associated with Accretion Disks of Type II Collapsars. The Astrophysical Journal 778:8.
[doi:10.1088/0004-637X/778/1/8]

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